The present invention relates to a new distillation process for working-up aqueous amine solutions.
It is well known (See, for example, DE-OS Nos. 1,542,544, 1,947,851, 2,106,644, 2,135,154, 2,214,056, 2,456,308; BE-PS Nos. 631,946, 661,047, 661,946; FR-P No. 1,359,438 and GB-P No. 768,111) that it is possible to produce aromatic diamines by catalytic hydrogenation of the appropriate aromatic dinitro compounds. The hydrogenation can take place with the aid of solvents such as alcohols which boil at a low temperature (e.g., methanol, ethanol, or isopropanol) and also without such solvents. The hydrogenation may be carried out with catalysts which are dispersed in the reaction mixture during the reaction and then separated off by sedimentation or filtration and, as necessary, re-used.
Up to now, the working-up of such reaction mixtures has taken place in such a way that the mixture of aromatic diamines and reaction water present after any adjuvant solvent used has been separated off is continuously freed of water under normal pressure in a distillation column. The diamine accumulates as a distillation residue and water which adheres to the diamine and any organic impurities which might still be present are separated from the residue in additional steps. Mixtures of water with organic by-products which are volatile in steam, such as those that accumulate in the hydrogenation of the dinitro aromatic substances arise as distillates. Examples of such by-products are aromatic or cycloaliphatic monoamines and/or cycloaliphatic alcohols. For example, in the case of the production of diaminotoluene, toluidines, perhydrotoluidines and/or methyl cyclohexanols form as by-products.
These by-products which are volatile in steam have the effect of causing the water distilled off overhead to be loaded heavily with these compounds. Removal of those by-products from the water is possible only at great expense, for example, by using a biological clarifier.
It is an object of the present invention to make available a new process for the working-up of aqueous amine solutions in which the waste water contains less organic impurities.
This and other objects which will be apparent to those skilled in the art are accomplished by the process of the present invention which makes it possible for the energy which has been spent on evaporating the water to be largely reclaimed by using the heat of condensation of the vapors.